This invention relates to fluorescent lamp power supplies. More particularly, this invention relates to a fluorescent lamp power supply and control circuit which enables the lamp to be regulated to shine at a substantially constant intensity as the lamp ages or the power supply voltage fluctuates, and which also enables lamp intensity to be adjusted continuously and smoothly over a chosen intensity range including, if desired, substantially from full OFF to full ON.
Fluorescent lamps are finding increased use in systems requiring an efficient and broad-area source of visible light. For example, portable computers such as lap-top and notebook computers use fluorescent lamps to back-light or side-light liquid crystal displays to improve the contrast or brightness of the display. Fluorescent lamps have also been used to illuminate automobile dashboards, and are being considered for use with battery-driven backup emergency EXIT lighting systems in commercial buildings.
Fluorescent lamps find use in these and other low-voltage applications because they are more efficient, and emit light over a broader area, than incandescent lamps. Particularly in applications requiring long battery life, such as in the case of portable computers, the increased efficiency of fluorescent lamps translates into extended battery life or reduced battery weight, or both.
In low-voltage applications such as those discussed above, a power supply and control circuit must be used to operate the fluorescent lamp. This is because power typically is provided by a 3-20 volt DC source, while fluorescent lamps generally require 100 volts AC or more to efficiently operate. Accordingly, a power supply and control circuit is needed to convert the available low DC voltage into the necessary high AC voltage.
Previous known fluorescent lamp power supply and control circuits have suffered from one or more drawbacks. Some circuits, for example, cannot smoothly and continuously vary the intensity of a fluorescent lamp from substantially full OFF to full ON. These circuits have low intensity "dead-spots" which cause the fluorescent lamp to either abruptly and prematurely turn OFF when the lamp's intensity is reduced toward zero, or to abruptly "pop-on" when the intensity is increased from zero. Other known circuits avoid this problem simply by limiting the range over which the lamp's intensity can be varied. These circuits do not allow adjustment of intensity over the range of full OFF to full ON.
A further disadvantage of some previous known fluorescent lamp power supply and control circuits is that lamp intensity may change as the lamp ages or as the power supply voltage fluctuates.
Yet another disadvantage of some previous known fluorescent lamp power supply and control circuits is that they are inefficient. This inefficiency necessitates the use of larger and heavier batteries or results in decreased battery life. Neither is desirable in portable computer applications.
A further disadvantage of some known fluorescent lamp power supply and control circuits is that they can be a source of radio frequency emission. Such emission can cause undesirable electromagnetic interference with nearby devices, and can degrade overall circuit efficiency.
An additional disadvantage of some known fluorescent lamp power supply and control circuits is that at relatively low intensity levels, low excitation voltages and currents associated with the fluorescent lamp can result in an electromagnetic field that is non-uniformly distributed along the length of the fluorescent tube. Consequently, the light output degrades along the length of the tube, typically with incomplete or no visible output at the low voltage end of the tube. Previous known circuits that address non-uniform light distribution typically include voltage mode regulation circuitry floating from the lamp. Unfortunatly, the voltage mode regulation causes the range of dimming to be limited, and thus, the lamps have narrow operating ranges.
In view of the foregoing, it would therefore be desireable to provide a power supply and control circuit for a fluorescent lamp which enables the lamp's intensity to be regulated so that it shines at a substantially constant intensity as the lamp ages or as the power supply voltage fluctuates.
It would also be desirable to provide a power supply and control circuit for a fluorescent lamp which enables the lamp's intensity to be continuously and smoothly adjusted by a user over a chosen range of intensities.
It would further be desirable to provide a power supply and control circuit for a fluorescent lamp which enables the lamp's intensity to be continuously and smoothly adjusted by a user from substantially full OFF to full ON.
It would additionally be desirable to be able to provide such a fluorescent lamp power supply and control circuit which is efficient, and which produces a minimum of spurious radio frequency emissions.
It would still further be desirable to provide a fluorescent lamp power supply and control circuit which enables the lamp to generate light output which is uniformly distributed throughout the length of the fluorescent tube for a wide range of operating parameters.